1. Field of the Invention
The present invention relates to a three dimensional measurement apparatus and a method using a space encoding method for generating spatial modulation by projecting pattern lights including bright parts and dark parts arranged at an arbitrary width.
2. Description of the Related Art
In a well known measurement method in three dimensional measurement, a projection apparatus and an imaging apparatus are arranged in a known relationship. The projection apparatus projects pattern lights to a subject, and the imaging apparatus images the subject. The principle of triangulation is used based on the relationship between the projection apparatus and the imaging apparatus to calculate the distance to the subject. For example, there is a method of three dimensional measurement as in Japanese Patent Laid-Open No. 2007-192608. In the method, a first bright and dark pattern light including bright parts and dark parts alternately arranged at an arbitrary width and a second bright and dark pattern light in which the phase of the first bright and dark pattern light is shifted are projected. Tone distributions of photographed images are acquired, and positions of intersections between the first and second pattern lights on the imaging element are calculated.
It is fundamental in ranging to calculate the positions on the imaging element of the intersections calculated from the first and second pattern lights. It is more important to correctly calculate intersection distances of the first and second bright and dark pattern lights than calculate absolute values of the intersection positions. This is because although the absolute values of the intersections are necessary to calculate the absolute position of the specimen, the shape of the specimen is mainly measured in the three dimensional measurement, and in that case, it is only necessary to accurately calculate the relative positional relationship between the intersections. The measurement of the absolute position of the specimen can be attained by preparing a known index of the absolute position and measuring, i.e. calibrating, the relative relationship between the index and the specimen.
In a general intersection detection method, the tone distributions of the first and second bright and dark pattern lights on the imaging element are sampled by imaging pixels, and straight line approximation is applied to the sampling points of the pattern lights to calculate the intersections. When a period of the patterns projected to the measurement target object is sampled by relatively few imaging element pixels, the intersections include sampling errors due to the straight line approximation if the intersections are calculated by the conventional method. Therefore, there are errors in the intersection distances if the intersection distances are calculated from the intersections.
If a period of the pattern lights is sampled by many pixels of the imaging element, the errors in the intersections and the errors in the intersection distances can be reduced. However, a high-resolution imaging element is necessary, and this leads to an increase of the apparatus size and cost.